Field of the Invention
The present invention relates to an air conditioner for a vehicle.
Description of Related Art
In electric vehicles, because the engine cooling water cannot be used to heat the vehicle interior, an air conditioner for a vehicle that utilizes a heat pump cycle is employed (see, for example, Japanese Unexamined Patent Application Publication (JP-A) Nos. 5-310034 and 2000-343934). In addition to a compressor, this air conditioner for a vehicle is provided with a heating expansion valve and an external heat exchanger, and also with a cooling expansion valve and an internal heat exchanger.
During a warming operation, a heat exchange medium that has been compressed by the compressor is expanded by the heating expansion valve, and flows into the external heat exchanger. As a consequence, the heat exchange medium absorbs heat via the external heat exchanger, and then releases this heat in the internal heat exchanger so as to warm the interior of the vehicle compartment.
During a cooling operation, a heat exchange medium that has been compressed by the compressor is expanded by the cooling expansion valve, and flows into the internal heat exchanger. As a consequence, the heat exchange medium absorbs heat in the internal heat exchanger so as to cool the interior of the vehicle compartment, and then releases this heat in the internal heat exchanger.
In this air conditioner for a vehicle, because the heat exchange medium absorbs heat during a warming operation in the external heat exchanger, if the external temperature is extremely low, then in some cases, frost formation may occur on the surface of the external heat exchanger. If frost formation does occur, then because the heat transfer rate deteriorates so that the heat absorption is inadequate, the problem arises that the warming effect inside the vehicle compartment is insufficient.
Note that, during a cooling operation, because heat is released in the external heat exchanger, performing an operation to defrost the external heat exchanger by switching from a warming operation to a cooling operation may be considered. However, in a defrosting operation that requires this type of cooling operation to be performed, cold air is supplied to the vehicle interior so that it becomes extremely uncomfortable for occupants of the vehicle.
Therefore, in the technology disclosed in Japanese Unexamined Patent Application Publication (JP-A) Nos. 5-310034 and 2000-343934, an operation to defrost the external heat exchanger is performed using hot gas. In this defrosting operation, the heat exchange medium (i.e., the hot gas) that has been compressed by a compressor bypasses the expansion valve before flowing into the external heat exchanger. Because this heat exchange medium is able to release heat without absorbing heat in the external heat exchanger, it is able to defrost the external heat exchanger. Moreover, because it also does not absorb heat in the internal heat exchanger as well, cold air is not supplied to the vehicle interior.
Moreover, conventionally, an air conditioner for a vehicle is known that, for example, in an air conditioner that performs a defrosting mode operation using a heat pump cycle, is provided with a temperature sensor that detects the surface temperature of the external heat exchanger, and determines whether or not the external heat exchanger is in a frosted state based on the detection value of the surface temperature of the external heat exchanger, and then performs a defrosting mode operation based on the result of this determination (see, for example, Japanese Unexamined Patent Application Publication (JP-A) No. 2010-236709).